The goal of this book is to develop a working hypothesis for mitral valve function in the beating heart. We have been studying the 4-D dynamics of the heart using biplane radiography of surgically implanted radiopaque markers for the past forty years,1 with emphasis on the mitral and aortic valves during the past 20 years, and dense leaflet and annular marker arrays during the past several years. Data from the control runs in these studies comprise the substrate for this book.